Preheat oven to 325 degrees. Separate egg whites in a stainless steel bowl and let them come to room temperature. In a separate medium bowl, sift gluten free flour, salt, xanthan gum, and 3/4 cup of sugar. Beat the egg whites until foamy. Add the cream of tartar. Beat until the egg whites form soft peaks. Beat in the other reserved 3/4 cup of sugar about 3 Tablespoons at a time. Beat until stiff peaks form. Slowly fold in flour mixture about 1/4 cup at a time. Once the flour mixture is combined, fold in the vanilla. Slowly pour the batter into a tube pan (preferably one where the inside lifts out) and spread the batter evenly. Run a knife through the cake to eliminate any air bubbles. Bake for 50-55 minutes until top is golden and sides begin to pull away. Cool for 10 minutes. Run the edges of a knife along the pan and pull out the insert to the tube pan. Run a knife along the bottom of the insert and invert onto a cake plate. Serve with topping of choice!